This invention relates to semiconductor devices operative at relatively high frequencies and, more particularly, to a high electron mobility transistor (HEMT) semiconductor device structure for achieving both high gain and high linear power operation at such frequencies. Specifically, the invention is directed to a pseudomorphic MODFET having an HEMT structure which preferably comprises an n+ GaAs layer as the carrier supply layer, instead of a conventional n+ Al.sub.x Ga.sub.1-x As or n+ Al.sub.x In.sub.1-x As carrier supply layer. The MODFET in accordance with the invention exhibits improved transconductance, g.sub.m, at high gate bias and provides improved large signal linear power performance.
During the past several years, MODFETs having various structures have been studied extensively due to their high frequency performance relative to other semiconductor devices. MODFETs can achieve very high P.sub.sat due to their high carrier density and carrier velocity. However, the large signal linear power performance of MODFETs, such as their power at a -1 dB gain compression point, P.sub.1db, and their harmonic/intermodulation power levels, is typically less than desired. See, for example, B. Kim, et al., "mm-Wave AlGaAs/InGaAs/GaAs Quantum Well Power MISFET", IEDM Tech. Dig., December 1988, pp. 168-171, and L. F. Lester, et al., "High Efficiency 0.25 .mu.m Gate-Length Pseudomorphic Power Heterostructure FET's at mm-Wave Frequencies", 47th DRC Tech. Dig., June 1989, IVB-4.
FIG. 1 shows the small signal level equivalent circuit model for an FET. Under large signal levels, however, the values of almost all of the equivalent circuit parameters vary as functions of the gate bias, V.sub.gs, and the drain bias, V.sub.ds. Those variations cause gain compression, as well as increase the harmonic power levels, as the input power increases. See S. M. Perlow, "Basic Facts About Distortion and Gain Saturation", Applied Microwave, May 1989, pp. 107-117. FIG. 2 shows the output power and gain compression for a typical MODFET as the input power level increases. The major sources of nonlinear behavior in FETs under large signal conditions have been identified as g.sub.m, g.sub.o, and C.sub.gs variations as functions of the biases. See G. Lambrianou, et al., "Power Characterization of a MESFET Amplifier Using Small Signal Measurement and Volterra Series", IEEE MTT-S Digest, 1985, pp. 409.varies.412, and R. E. Williams, et al., "Graded Channel FET's: Improved Linearity and Noise Figure", IEEE Trans. on Elect. Dev., Vol. ED-25, No. 6, June 1978, pp. 600-605.
g.sub.m variation as a function of the gate voltage swing relates to the output power linearity of the FET. Consequently, the power/gain saturation curve for a conventional MODFET is usually soft, and P.sub.1db is not very high despite high saturation power due to maximum drain current. Also, the donors in the carrier supply layer can become partially ionized at high gate bias due to the relatively low donor energy level in the material. This partial ionization, together with the presence of the DX centers in the heavily doped AlGaAs or AlInAs material, limits the total carrier concentration in the device and degrades the modulation efficiency. See P. M. Mooney, et al., "The Role of DX Centres in Limiting the Free Carrier Density in GaAs", Proc. of Int. Symp. GaAs and Related Compounds, 1987, pp. 359-362, and M. C. Foisy, et al., "The Role of Inefficiency Charge Modulation in Limiting the Current-Gain Cutoff Frequency of the MODFET", IEEE Trans. on Elect. Dev., Vol. 35, No. 7, July 1988, pp. 871-878.
Degradation in g.sub.m at high gate bias has been found to be the dominant limiting factor for the power/gain saturation performance in MODFETs. See M. R. Weiss, et al., "An Investigation of the Power Characteristics and Saturation Mechanisms in HEMT's and MESFET's", IEEE Trans. on Elect. Dev., Vol. 35, No. 8, August 1988, pp. 1197-1206. This g.sub.m degradation also limits the digital applications of MODFETs, where the signal swing is always large.
The g.sub.m degradation at high gate bias occurs because almost all of the MODFET structures on GaAs substrates utilize an n+ AlGaAs or n+ AlInAs layer as the carrier supply layer. This carrier supply layer becomes conducting at high gate bias. See K. Lee, et al., "Parasitic MESFET in (Al, Ga)As/GaAs MODFET's and MODFET Characterization", IEEE Trans. on Elect. Dev., Vol. ED-31, No. 1, January 1984, pp. 29-35. Such a parasitic conduction, in parallel with the 2-D channel conduction, is undesirable, because the inferior transport property in the carrier supply layer results in low carrier mobility and velocity, which in turn cause low g.sub.m, hence, low f.sub.T and f.sub.max at high gate bias according to Eq. 1 and Eq. 2, respectively: ##EQU1## where C.sub.gs is the gate capacitance, .nu..sub.s is the average saturation velocity, L.sub.g is the gate length, g.sub.o is the drain-source conductance, and R.sub.in is the input resistance of the MODFET.
Efforts have been made to achieve a more constant g.sub.m profile by providing a multiple quantum well structure (MQW) or doped channel MISFET structure. However, the MQW structure typically has lower gain, possibly due to the poor material growth compatibility; while the MISFET structure suffers low gain due to the degraded carrier velocity in the heavily doped channel. See G. W. Wang, et al., "A High-Current Pseudomorphic AlGaAs/InGaAs Double Quantum-Well MODFET", IEEE Elect. Dev. Lett., Vol. 9, No. 1, January 1988, pp. 4-6, and P. Sannier, et al., "A Double-Heterojunction Doped-Channel Pseudomorphic Power HEMT with a Power Density of 0.85 W/mm at 55 GHz", IEEE Elect. Dev. Lett., Vol. 9, No. 8, August 1988, pp. 397-398.